1. Field of the Invention
The present invention relates to an image pickup apparatus and an image pickup method therefor as well as an image pickup program implementing the method and a storage medium storing the program, and more particularly to an image pickup-apparatus equipped with a shake correction function and an image pickup method therefor as well as an image pickup program implementing the method and a storage medium storing the program.
2. Description of the Related Art
Some image pickup apparatuses, such as cameras or video cameras, incorporate an optical shake correction device so as to exert a shake correction function. Shake correction by this shake correction device is performed by driving a shift lens as a part of taking lenses in a direction perpendicular to the optical axis of the taking lenses to thereby change the optical axis of the taking lenses.
FIG. 8 is a block diagram schematically showing the arrangement of an image pickup apparatus incorporating the conventional shake correction device.
Originally, the correction of a shake of an image pickup apparatus is performed in two directions, i.e. a horizontal direction and a vertical direction, on a plane perpendicular to the optical axis of the taking lenses of the image pickup apparatus. For simplicity of explanation, however, there is shown in FIG. 8 the image pickup apparatus configured to perform shake correction only in the horizontal direction.
As shown in FIG. 8, the image pickup apparatus 800 is comprised of an angular velocity sensor 101′ that outputs an angular velocity signal representative of a shake of the image pickup apparatus 800, a high-pass filter (HPF) 102′ that eliminates a drift and other undesired components of the angular velocity signal output from the angular velocity sensor 101′, an amplifier 103′ that amplifies the angular velocity signal output from the angular velocity sensor 101′, a camera system control microcomputer (hereinafter simply referred to as “the microcomputer”) 120′ that controls camera functions including shake correction, autofocus (AF), zoom, and automatic exposure (AE), a shift lens 133′ as a part of taking lenses, not shown, a position sensor 115′ that detects the position of the shift lens 133′ to output a position signal, an amplifier 116′ that amplifies the position signal output from the position sensor 115′, and an H bridge driver 114′ that drives the shift lens 133′ horizontally on a plane perpendicular to the optical axis of the taking lenses.
The microcomputer 120′ is comprised of a built-in A/D converter 104′ that converts the angular velocity signal into a digital signal to provide angular velocity data, an HPF 105′ and a phase compensation filter 106′ that perform predetermined signal processing on the angular velocity data, a variable HPF 107′ that is capable of changing a cut-off frequency for panning control, described in detail hereinafter, an integrator 108′ that generates an angular signal to provide a correction signal as a target drive value for driving the shift lens 133′, a built-in A/D converter 117′ that converts the position signal amplified by the amplifier 116′ into a digital signal to provide position data, an adder 111′ that calculates the difference between the current position of the shift lens 133′ and the target drive value to output an actual correction amount, a low-pass filter (LPF) 112′ that filters the output from the adder 111′ to reduce drive noise generated by the H bridge driver 114′, and a pulse width modulator (PWM) section 113′ that performs pulse width modulation (PWM) on the output from the LPF 112′.
The microcomputer 120′ drives the shift lens 133′ on a plane perpendicular to the lens optical axis by the H bridge driver 114′ in accordance with-the PWMed output from the PWM section 113′, to thereby perform shake correction.
In the following, a description will be given of the panning control which is executed by the image pickup apparatus 800.
When a photographer performs a shooting operation involving movement of the image pickup apparatus 800, i.e. a panning operation, it is preferable that an image is moved as intended by the photographer. However, when normal shake correction is performed during the panning operation, an image is suppressed from moving at the start of the panning operation due to execution of the shake correction, and only after a possible shake correction range is exceeded, the image suddenly starts to move. This makes image motion discontinuous. Further, at the end of the panning operation, a phenomenon occurs in which the shake correction is suddenly limited at a correction end so that an image is fixed at the correction end, i.e. a correction end-hitting phenomenon (a state where the shift lens cannot be shifted any more) occurs, which makes it impossible to perform shake correction. To avoid this inconvenience, when the amount of correction by panning control becomes large, DC components of the output of the integrator 108′ are cut off so that shake correction device of the image pickup apparatus 800 is centered on a central part of the possible shake correction range.
The panning control includes, by way of example, changing the cut-off frequency of the variable HPF 107′ when the output from the integrator 108′ exceeds a predetermined correction range, so as to eliminate low-frequency signal components of the integrator output to thereby limit the amount of shake correction. During the panning operation, this panning control makes the shake correction signal, indicative of a target position, closer to a central position of the image pickup area, thereby eliminating the above-mentioned inconvenience.
FIG. 9 is a diagram showing changes in the HPF cut-off frequency (Hz) dependent on changes in the magnitude of the output from the integrator in FIG. 8.
In FIG. 9, a line graph shows the relationship between the magnitude of the output from the integrator 108′ and the HPF cut-off frequency (Hz). When the output from the integrator 108′ exceeds a set value A, the HPF cut-off frequency (Hz) of the variable HPF 107′ becomes higher with an increase in the output from the integrator 108′. Further, when the output from the integrator 108′ exceeds a set value B, the HPF cut-off frequency (Hz) rises more sharply so as to prevent occurrence of the correction end-hitting phenomenon.
The amount of shake correction is limited during the panning operation by changing the HPF cut-off frequency (Hz) of the variable HPF 107′ according to the magnitude of the output from the integrator 108′, as described above, whereby an image can be moved as intended by the photographer.
In the case where a-still image is taken using the image pickup apparatus having the above described shake correction function, it is preferred that the shake prevention effect is higher so as to pick up an unblurred image. However, when a camera shake occurs, panning control is sometimes executed, which can cause degradation of the shake prevention effect. To prevent the degradation of the shake prevention effect, a method has been proposed in which the panning control is switched between moving image shooting and still image shooting (see e.g. Japanese Laid-Open Patent Publication (Kokai) No. 2002-209136). Further, another method has been proposed in which the panning control is inhibited during still image shooting (see e.g. Japanese Laid-Open Patent Publication (Kokai) No. 2002-359768). These methods are now used to enhance the shake prevention effect during still image shooting.
However, in the method in which the panning control is switched between moving image shooting and still image shooting, it becomes impossible, depending on the amount of limitation of the shake correction by panning control, to obtain a shake prevention effect required during a low-shutter speed operation. Further, if the amount of limitation of the shake correction by panning control is too small or panning control is not executed at all, e.g. when panning is intentionally performed by a photographer with a view to panning an image, the correction end-hitting phenomenon can occur, which causes a still image to be blurred or hinders the photographer from shooting an image having a composition intended by him/her.